Photochemistry and Photophysics of Coordination Compounds

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176 S. Campagna et al. signature. This characterization was made by extensive work (including spectroelectrochemistry in various solvents, as well as at different pH values in aqueous solution) to identify the intermediates and to clarify the effect of pH on the processes [315–317]. Indeed, it has been demonstrated that in some solvents, processes 2 and 4 take place simultaneously, so it would be more correct to talk of proton-coupled electron transfer instead of successive electron/proton transfer events. At pH 4, only a fully reversible, coupled twoelectron/two-proton transfer process is observed for 51. This is a rare example of a proton-coupled multielectron transfer reaction [317] (FM MacDonnell, personal communication). Another interesting example of photoinduced multielectron collection, this time at a metal center rather than at a ligand site, has been reported for a series of trimetallic mixed-metal species [318–320]. The most recent example of the series is 53 [320]. In this species, two Ru(II) chromophores (the light absorber units, LA) are linked to one Rh(III) center, which represents the electron collection (EC) core, through polyazine bridging ligands (BLs). The absorption spectrum of this species is dominated by the absorption of the Ru LA subunits, while the reduction properties identify the Rh(dσ ∗ ) orbital as the LUMO [319]. Upon excitation of the peripheral Ru(II) chromophore, an oxidative electron transfer to the Rh(III) center takes place (k = 1.2 × 10 8 s –1 ), populating a triplet Ru(II)-to-Rh(III) charge transfer state. In the presence of a sacrificial donor, dimethylaniline, which restores the Ru(II) center(s), the starting compound undergoes a net photoreduction process with formation of [{(bpy)2Ru(dpp)}2Rh I ] 5+ , as also demonstrated by spectroelectrochemistry, in which two chlorides have been lost (probably by irreversible chlo-
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 177 ride oxidation), and a two-electron reduced unit Rh(I) is formed [320]. The Rh center should simultaneously undergo a structural reorganization (most likely, octahedral/square planar). Interestingly, the photoreduced species is coordinatively unsaturated and therefore could be available to interact with substrates. It warrants mentioning that photoinitiated electron collection is obtained in a trimetallic species having an Ir(III) redox-active center instead of a Rh(III) one and similar Ru-based LA units [318]. 5.7 Photoinduced Multihole Storage: Mixed Ru–Mn Complexes Complementary to the topic discussed in the previous section (that is, to accumulate multiple electrons in a single site of a (supra)molecular species) is the development of systems capable of accumulating holes, as happens in the oxygen evolving systems of natural photosynthesis. The source of inspiration is the photosystem II [321], where the excited primary chlorophyll donor, ∗ P680, one of the most effective photooxidants of natural systems, is able to extract up to four electrons in consecutive steps from the so-called manganese cluster, whose structure—at least for a specific natural system—has recently been revealed [322–324]. The four-times oxidized manganese cluster successively produces molecular oxygen, thus returning to its initial state, ready for another photoinduced catalytic cycle. Since the inspiration is photosystem II, it is not surprising that the largest family of complexes made to photochemically accumulate “holes” are Ru(II) polypyridine complexes coupled to manganese species. The field has been recently reviewed [325]. Several Ru–Mn dyads were initially studied to investigate some specific parameters for electron transfer (see for example 54–57 [326]). In a typical experiment, the excited Ru(II) chromophore is quenched via a bimolecular oxidative electron transfer by a sacrificial acceptor (usually methyl viologen), and the oxidized Ru(III) species oxidizes the attached Mn(II) subunit to Mn(III). Time constants of these latter processes spanned a large range, from
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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